Wireless transceivers, such as those used in wireless utility meter reading, often utilize a radio frequency (RF) signal to send data between a transmitter and a receiver. In an ideal frequency modulated (FM) system, the frequency of the transmitter and the receiver are matched. Even a slight difference in frequency can lead to a reduction in system performance.
Both cost and accuracy of a system suffer as a result of compensating for frequency shifting between the transmitter and the receiver. In order to keep the costs down, less expensive oscillator circuits are often used. Errors in frequency can result from temperature shifts, aging, mechanical shock, and manufacturing tolerances. These errors tend to be randomly distributed and change over time. Consequently, frequency accuracy may drift, and if the drift is too great, then the receiver may not be able to decode the incoming signals.
Typically, to compensate for the frequency difference of a transmitter and its corresponding receiver, either a system is designed with stable and accurate reference oscillators, which in many cases are expensive, or the bandwidth of the system is widened so that the shifts in frequency fall within the bandwidth of the receiver. Widening the bandwidth can lower the signal-to-noise ratio (SNR) of the system, which, in turn, decreases the performance of the system. The SNR measures the strength of a signal relative to the background noise and is measured in decibels (dB). Maximizing the SNR can increase the performance of the system.
Specific modulation schemes compensates for the shift in frequency between the transmitter and the receiver. One modulation scheme, called quadrature modulation, subdivides a binary data stream into pairs of two bits and represents each pair with one of four levels before performing modulation. Quadrature Phase Shift Keying (QPSK) refers to a type of quadrature modulation in which two data bits represent four different possibilities: 0 degrees, 90 degrees, 180 degrees, and −90 degrees. These four levels correspond to positions around the unit circle when the unit circle represents phase. One drawback of QPSK is that large phase changes at the end of each symbol (pair of two data bits) can lead to undesirable transitions if the waveform is to be filtered and subsequently processed by a nonlinear power amplifier.
Another popular type of modulation is binary frequency shift keying (BFSK). In BFSK, binary baseband data selects one of two carrier frequencies with equal amplitudes: one carrier frequency corresponds to a “1” and the other carrier frequency corresponds to a “0”. In effect, the “frequency” is “shifted” to “key” the data. Since the frequency shifts or “keys” between two frequencies, the process is referred to as binary frequency shift keying.
Automatic utility meter reading represents one application that employs wireless transceivers. While these transceivers employ QPSK or BFSK, they suffer from frequency shifting; therefore, stable frequency sources are necessary.